Chemical-mechanical polishing (CMP) is a widely used means of planarizing silicon dioxide as well as other types of surfaces on semiconductor wafers. Chemical mechanical polishing typically utilizes an abrasive slurry disbursed in an alkaline or acidic solution to planarize the surface of the wafer through a combination of mechanical and chemical action.
One type of chemical mechanical polishing system has a rotatable circular platen or table on which a polishing pad is mounted. A multi-head polishing device is positioned above the table. The polishing device has multiple rotating carrier heads to which wafers can be secured typically through the use of vacuum pressure. In use, the platen is rotated and an abrasive slurry is disbursed onto the polishing pad of the platen. Once the slurry has been applied to the polishing pad, the rotating carrier heads move downward to press their corresponding wafers against the polishing pad. As the wafer is pressed against the polishing pad, the surface of the wafer is mechanically and chemically polished.
FIG. 1 illustrates a typical polishing pad 120 shown mounted on a platen 110. The polishing pad 120 includes a bottom pad 122 mounted on the platen and a top pad 124 mounted on the bottom pad. Typically, the top pad 124 is adhered to the bottom pad 122 using a glue. The bottom pad 122 serves as a damper and typically is formed from foam or felt. The top pad 124 generally contacts the wafer for polishing and is typically formed from polyurethane.
Polishing pads, such as the one described above, are engineered in an effort to maximize their planarization efficiency. Simultaneously, efficient, extended use of the pads generally requires pad conditioning between polishing runs. Conditioning typically includes applying a conditioning tool, such as a diamond impregnated steel plate, to the top pad to remove expired surface and expose fresh pad material. Repeated conditioning leads to thinning of the pad, a resultant decrease in the planarization efficiency and eventual end of useful pad life.
At present, a variety of methodologies are used to determine the appropriate time for a pad change. These include: number of wafers polished, number of pad condition hours, direct measurements of pad thickness. The first two methods are easily employed but prone to error due to the indirect nature of the measurements. The third method is inconvenient and labor intensive.